1. Field of the Invention
The present invention relates to a grinding method and apparatus capable of grinding a workpiece at high shape accuracy by controlling the rotation of the workpiece.
2. Discussion of the Related Art
Generally, in a grinding operation of a slender workpiece, the grinding resistance acting on a grinding wheel causes the workpiece to flex or yield when being grater than the rigidity of the workpiece. With the flexure, the workpiece is deviated from a grinding center, and an infeed amount of the grinding wheel against the workpiece cannot be a programmed grinding allowance. A problem arises from this in that the deviation of the workpiece causes errors in dimension as well as shape accuracy (e.g., roundness).
To cope with the aforementioned problem, as shown in FIGS. 8 and 9, steady rest devices 70 are provided at places which are selectively brought to face a grinding wheel 72, to enhance the machining accuracy by preventing the workpiece 74 from flexing.
However, the steady rest devices 70 need fine adjustments each time the kind of the workpieces is changed, so that a lot of time is required in altering the setup for the workpieces of a new kind. Further, for the grinding of a cylindrical workpiece 74 with the steady rest devices 70, the positions of rest shoes have to be controlled to follow the workpiece whose diameter changes with the progress of the grinding operation, thereby making the steady rest devices 70 complicated and expensive.
Therefore, a grinding machine disclosed in Japanese unexamined, published patent application No. 2-124254 has been developed as a technology for performing a grinding operation without using any such steady rest device. In the technology, there are used rotational position detecting means for detecting the rotational position of a workpiece and grinding resistance altering means for altering the grinding resistance, and by these means, the grinding resistance is controlled to be decreased at each rotational phase in which a grinding part (i.e., arc segment) is narrowed in the substantial grinding width due to the opening an oil hole thereon, so that each grinding part with the oil hole opening thereat can be prevented from being infed deeper than other non-holed parts (arc segments) of the workpiece. In this case, there can be conceived of various grinding resistance altering means for altering the grinding resistance upon grinding each local area at which the oil hole 6 opens. The various means include means for varying the rotational speed of the workpiece about the axis of the same or the rotational speed of the grinding wheel, means for varying the infeed amount of a wheel head, or means for varying the compliance of bearings which rotatably support the workpiece or grinding wheel bearings.
For a cylindrical workpiece with the oil hole opening at known angular phases, the technology described in the Japanese application has been designed to prevent each arc part with the oil hole from suffering a deep infeed by partially controlling the fluctuation of the grinding resistance caused by the oil hole for the period of each particular angular phase. However, nothing is described in the Japanese application about a grinding method of enhancing the shape accuracies in respective rotational phases of a workpiece of the property that the continuous variation in rigidity causes the workpiece to be overcut or undercut locally on the circumference thereof.